Diagnostic imaging serves an important role in the field of ophthalmology and the utilization of optical coherence tomography (OCT) has advanced in recent years. OCT is used not only for acquisition of B-mode images and three dimensional images of a subject's eye but also for construction of front images (also referred to as en-face images) such as C-mode images and shadowgrams, for construction of blood vessel emphasized images (also referred to as angiograms), for blood flow measurement, for evaluation of the size and/or morphology of an ocular tissue, and the like.
A cornea is a typical ocular tissue that can be evaluated using OCT. For example, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2015-504740 discloses a technique for evaluating an anterior eye segment parameter such as corneal thickness and curvature. The technique can be used for checking time-dependent change of the cornea in evaluation of surgery, evaluation of treatment, prognosis management, and the like. Checking the time-dependent change is performed, for example, by comparing measurement results (e.g., corneal thickness map) obtained at mutually different times. An example of the mutually different times includes a time before surgery and a time after the surgery.
In such comparative evaluation, the accuracy of registration of maps is important. In order to satisfy this requirement, it is conceivable to increase the precision of alignment at the time of measurement to ensure the reproducibility of the measurement position. However, there are problems in this method such as the followings: the alignment may take a long time; the alignment may be affected by involuntary eye movements etc.; the reference of alignment (e.g., corneal apex) may deviate due to deformation of ocular tissues by having undergone surgery.
In this regard, the technique described in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2015-504740 makes it possible to perform inter-map registration by specifying the corneal apex position from an approximate curved surface of the corneal shape obtained using OCT even if alignment at the time of measurement is deviated to a certain degree. However, the conventional technique cannot be employed in an appropriate manner for comparative observation before and after surgery that may bring about cornea deformation.